Power storage devices typified by an electric double-layer capacitor and a lithium ion battery have attracted attention for their large capacitance. The performance of such power storage devices largely depends on an electrode material, and electrode materials that can increase the electrostatic capacitance, the energy density, and the power density have been developed.
It has been known that graphene is used as an electrode material of an electric double-layer capacitor. For example, a sheet-shaped electrode including a porous graphene layer through which electrolyte solution ions can pass has been developed (see, for example, Non-Patent Literature 1). According to Non-Patent Literature 1, graphene is integrated into layers to achieve an energy density of 35 Wh/kg. Another example is a porous graphene sheet in which graphene is grown by CVD using MgO as a template (see, for example, Non-Patent Literature 2). According to Non-Patent Literature 2, the electrolyte solution ions move easily to a vertical direction of the graphene sheet to achieve an electrostatic capacitance of 303 F/g.
Still another known example is a graphene sheet film having carbon nanotube held between graphene sheets (see, for example, Patent Literature 1). According to Patent Literature 1, by using the conductivity of carbon nanotube in addition to the characteristics of the graphene sheet, an electrostatic capacitance of 290.6 F/g and an energy density of 62.8 Wh/kg are achieved.
An electrode material including graphene with nanopores has also been developed (for example, see Non-Patent Literatures 3 and 4, and Patent Literature 2). According to Non-Patent Literature 3, an electrode material in which nanopores are introduced to graphene by KOH using microwaves achieves an energy density of 100 Wh/kg. According to Non-Patent Literature 4, graphene oxide is separated by microwaves, activation is performed using KOH, and then, heating is performed, so that nanopores are introduced to graphene. Such graphene includes nanopores each having a size of 0.6 nm to 5 nm, and has a specific surface area of 3100 m2/g. According to Patent Literature 2, nanopores are introduced to graphene by a continuous electrolytic peeling method. In addition, Patent Literature 2 discloses that the electrode material includes a stack of carbon nanotube and graphene having nanopores introduced by a continuous electrolytic peeling method. Such an electrode material achieves an energy density of 90.3 Wh/kg.
On the other hand, it has been known that graphene is used for other purposes than the electrode material (for example, see Patent Literature 3 and Non-Patent Literature 5). Patent Literature 3 discloses that macromolecules are analyzed using a nanopore device based on graphene. Non-Patent Literature 5 discloses a sensor in which nanopores are introduced to a graphene stack and DNA is caused to pass the nanopores by an electric field.
As described above, it has been examined to utilize the characteristics of graphene by various perspectives and attempts of means for those perspectives. However, the examinations for the utilization of graphene are still in the unexplored territory, and it has been strongly expected to develop an electrode material including graphene that further improves capacitor characteristics such as the electrostatic capacitance, the energy density, and the power density. It would be preferable if other applications of such an electrode material were developed.